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Thread: How many Amps does a 5 HP motor need?

  1. #1

    How many Amps does a 5 HP motor need?

    I will be Hooking my compressor up to my fuse panel and need to buy a circuit breaker. It is a 5.0 HP 230 Volt single phase motor. I phoned customer support from the mfg, but am not convinced I got the right answer. They said to use a 40 amp breaker. The motor draws 22 amps from what I can see on the website. Just a gut feeling a 30 amp circuit is the right one. Anyone know for sure?
    Last edited by Bob Slater; 12-09-2008 at 7:31 PM.

  2. #2
    Hi Bob
    Use a 40 amp like they recommended if at all possible. The motor is probably rated for 22 amp continuous and will pull a lot more during starting. You might get by with a 30 amp breaker with only occasional trouble but why hassle with it?

  3. #3
    Remember that the breaker is to protect the wiring so if you're going to use a 40 amp breaker, make sure your wire in the circuit will handle 40 amps.

    Mike
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  4. #4
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    40A and 8 ga wire was what the folks at the elec supply house said to use for the contactor switch & breaker when I wired up my 5HP ClearVue cyclone.
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    Doug, How long was your run? I know you have to upsize wire for longer runs.

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    My 5hp Clear Vue is on 10 guage and 30 amp breaker. Never a problem. In fact, that was what Leeson recommended for the motor, IIRC. My run is only about 14' max from the breaker. Jim.
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    Quote Originally Posted by Jim O'Dell View Post
    My 5hp Clear Vue is on 10 guage and 30 amp breaker. Never a problem. In fact, that was what Leeson recommended for the motor, IIRC. My run is only about 14' max from the breaker. Jim.
    I agree with Jim......
    I would think #10 on double 30's would be fine.
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  8. #8
    I am pretty sure I ran 30 Amp Wire under the drywall, so I guess I better not go bigger than a 30 amp breaker. Will double check the wire tomorrow. Thanks for the replies. My table saw is also a 5 HP General and it is running fine on twin 20 amps breakers.

  9. #9
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    Quote Originally Posted by Andrew Nemeth View Post
    Doug, How long was your run? I know you have to upsize wire for longer runs.
    From the breaker box to the motor only about 20-22 ft. When the supply house consulted some tables and the info from the motor, they said I could go with 30A but said it was only about 2A from potentially popping the breaker. I'm no electrician and went with their recommendation. I think it was startup current draw they said might pop the breaker IIRC, not the normal operating current.
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  10. #10
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    I run my 5hp Woodmaster sander on 30amp/10ga without any problems but I would run a 5hp compressor on a 40amp/8ga due to the higher loads, particularly on startup.
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  11. #11
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    Quote Originally Posted by Bob Slater View Post
    ...It is a 5.0 HP 230 Volt single phase motor. ...
    A 30a circuit is fine for a 5HP motor. Unless you've got an extraordinarily long wire run 10ga wiring is also fine. The start surge on a 22 FLA motor won't trip a properly functioning 30a breaker unless the motor is locked up, otherwise defective, or is trying to start under a high static load. Modern compressors have unloader valves that reduce the startup loads so that it should be no harder starting than an equivalent motor spinning up a heavy blower wheel in a DC/cyclone.
    Last edited by Tom Veatch; 12-09-2008 at 8:36 PM.
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    Quote Originally Posted by Bob Slater View Post
    I will be Hooking my compressor up to my fuse panel and need to buy a circuit breaker. It is a 5.0 HP 230 Volt single phase motor. I phoned customer support from the mfg, but am not convinced I got the right answer. They said to use a 40 amp breaker. The motor draws 22 amps from what I can see on the website. Just a gut feeling a 30 amp circuit is the right one. Anyone know for sure?
    Hi Bob, check the motor nameplate for full load current, don't rely on the web site. You never know if the motor is a different model/manufacturer from the "typical" information presented on the site.

    Unless your motor is substantially different from what you said, a 30 Ampere breaker is the correct choice.

    Regards, Rod.

    P.S Asking how many amps a 5 HP motor needs isn't a question that can be answered. A high quality high power factor/high efficiency 5HP motor may be as low as 18 amperes, or as high as 28 if it's a low efficiency/low power factor motor. That's why you need to obtain the motor full load current from the motor nameplate....Regards, Rod.
    Last edited by Rod Sheridan; 12-10-2008 at 8:35 AM. Reason: Added post srcipt

  13. #13
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    A compressor has a much higher starting load unless it is empty. If it restarts under partial pressure you will want a 40 amp breaker and 8ga. wire. Why would you doubt the manufacturers recommendation??????

  14. #14
    Quote Originally Posted by Bruce Page View Post
    I run my 5hp Woodmaster sander on 30amp/10ga without any problems but I would run a 5hp compressor on a 40amp/8ga due to the higher loads, particularly on startup.
    Most big compressors have a bypass at startup, so they work the hardest when almost full.

  15. #15
    The manufacturer may recommend an oversized breaker just to minimize any hassles--it's no skin off their back if you spend more money than necessary. Also, a larger breaker doesn't necessarily mean a larger conductor.

    Note: the following is vastly simplified. Read the NEC for the fine print.

    You have two choices. You can treat it as a standard plug-and-cord connected device, in which case you need a conductor of 125% the full-load rating on the nameplate, and a breaker to match. If your motor is rated at 24A or less you can use #10 and a 30A breaker.

    Alternately, you can call it a dedicated motor circuit. The conductor ampacity requirement is the same as above. The advantage of calling it a motor circuit is that you're allowed to oversize the breaker to deal with startup surges. The normal limit for an inverse-time breaker is 2.5x the full-load rating, but if necessary this can be increased up to 4x the full-load rating.

    Yes, this does mean that the breaker is rated for more current than the conductors, but this is explicitly allowed for motor circuits.

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